1. Field of the Invention
The present invention relates to motor control systems for disk drive apparatus, and more particularly to control of actuator power to control head velocity during emergency head retraction.
2. Description of the Prior Art
Disk drives, particularly those used in portable and battery operated equipment, are subject to unexpected power system variations, such as loss of power supply and power failures. Damage to the magnetic disk as well as the read/write heads can occur during power loss conditions if a head crashes onto the disk. For disk drives using dynamic head loading the heads must be successfully unloaded to avoid such a crash. One method to unload the heads is described in U.S. Pat. No. 4,933,785 entitled "Disk Drive Apparatus Using Dynamic Loading/Unloading," which uses the rotational energy stored in the spin motor and disk assembly.
FIG. 1 provides a simplified block diagram of conventional head unloading system 100. The disk is controlled by a CPU (not shown) through CPU interface 180. Under normal operations actuator 120 is powered by actuator power source 160, while spin motor power source 170 drives spin motor 110. Switches 130a and 130b normally prevent any power from spin motor 110 from reaching actuator 120. During a power failure, switches 130a and 130b connect spin motor 110 with actuator 120 so that spin motor 110 acts as a DC generator. The electrical power generated by the back electromotive force (BEMF) of spin motor 110 is directed to actuator 120 through rectifier 150 and switches 130a and 130b. The heads (not shown), which are mounted on an actuator arm (not shown), are moved onto the unloading cam (not shown) into a parked condition. In addition, auxiliary power source 140 is used to power the logic devices used to control the unloading mechanism during a power failure.
The major problem with conventional unloading systems is that the design of a head unloading system requires a compromise between two conflicting conditions. If the heads are positioned near the outside diameter (O.D.) of the disk, the actuator travels only a short distance before hitting the unloading cam. Thus, a large actuator current is necessary to create the rapid acceleration needed to ensure that the heads reach a great enough velocity to climb the unloading cam. However if the heads are positioned near the inner diameter of the disk, the actuator travels a larger distance and builds up a greater velocity before hitting the unloading cam. The velocity of the actuator may be great enough to damage the heads when the actuator hits the outer crash stop. Typically, a resistor is inserted in series with the actuator motor to reduce the acceleration and velocity of the actuator. However, the insertion of the resistor further hampers the rapid acceleration needed when the heads are positioned near the outside diameter. The two conflicting conditions are explained in detail below with respect to FIGS. 2 and 3.
FIG. 2 shows the velocity vs time of the heads, which were located over an outer diameter of the disk, during a power failure condition. As explained above, if the heads are positioned near the outside diameter (O.D.) of the disk, the actuator travels only a short distance before hitting the unloading cam. In such a short distance, the actuator assembly does not have enough time to accelerate to a sufficient velocity to overcome the frictional forces encountered on the unloading cam. In addition, FIG. 2 also shows the actuator current vs time during the power failure. Time P marks the power failure condition; time U marks the instant when the heads make contact with the unloading cam; and time S marks the instant when the heads make contact with the outer crash stop. Until time U, the heads are accelerated to a velocity of approximately 22.64 inches per seconds (ips).
If the heads begin to climb the ramp with insufficient velocity, the heads may slow on the ramp surface and not be fully unloaded. As the heads climb the inclined portion of the unloading cam, the velocity of the heads is reduced to approximately 6.24 ips. During this time the heads are not "flying properly" because the suspension down-force has been partially removed. If the heads remain on the ramp surface for as little as 0.5 mS, the heads may pitch or roll and contact the disk surface which may damage the heads and the disk. Once the heads pass the inclined portion of the unloading cam, the heads begin to accelerate into the valley portion of the cam and finally come to rest against the outer crash stop at time S. Since a higher velocity is necessary to insure that the heads are not damaged, a high driving force on the actuator is desired when the heads are located near the outer diameter.
However, when the heads are located near the inner diameter of the disk, the actuator arm is a large distance away from the unloading cam. Therefore, the actuator arm accelerates to a high velocity before reaching the unloading cam. As shown in FIG. 3, the heads can reach a velocity of approximately 56 ips before the heads reach the unloading cam at time U. The head velocity is reduced slightly by the inclination of the unloading cam; however, the velocity remains approximately 47.4 ips upon impact with the outer crash stop. At that velocity, the heads bounce off the outer crash stop at a velocity of approximately 27.45 ips in the opposite direction before being forced against the outer crash stop by the actuator current. Such a rapid change of velocity is likely to damage the magnetic heads.
To reduce the speed of the actuator arm the voltage to the actuator assembly is reduced by placing a resistor 190 (FIG. 4) in series with actuator 120. However, reduction of the actuator voltage and actuator speed would increase the problem of inadequate speed if the magnetic heads are near the outer diameter of the disk when the power fails. Therefore optimization of the unloading mechanism requires contradictory actions depending on whether the magnetic heads are near the inner or outer diameter of the disk when a power failure occurs.
FIG. 5 shows a more detailed disk circuitry 500 containing an unloading mechanism. The disk circuitry is controlled by a CPU (not shown) through serial port 580. Under normal operations positive terminal Act+ of actuator 520 receives power from actuator amplifier 530. Negative terminal Act- of actuator 520 is coupled to a first end of the sense resistor 540. The second end of the sense resistor 540 is coupled to actuator amplifier 530.
Charge pump 502, which is used to supply bias voltages as required by spin motor amplifier 506, energizes auxiliary power supply 504, which is used only during power failures to power the head unloading circuits. Auxiliary power supply 504 is constructed to supply energy to the head unloading circuit for a much greater interval than is necessary to retract the heads from the disk, for example 60 mS. As shown in FIG. 5, a suitable auxiliary power supply can be constructed with three capacitors 504a, 504b, and 504c coupled in parallel between charge pump 502 and ground.
Three phase spin motor 509 is represented by inductors 508a, 508b, or 508c and a resistors 510a, 510b, or 510c. Three phase spin motor 509 is also coupled to the actuator through rectifier 150 and a field effect transistor (FET) 560.
For three phase spin motor 509, rectifier 150 is a three-phase rectifier comprising diodes 514a, 514b, and 514c which have their anodes coupled to grounds and their cathodes coupled to power phase a, b, and c respectively. Rectifier 150 further comprises diodes 516a, 516b, and 516c with their anodes coupled to power phases a, b, and c respectively and their cathodes coupled together to drain D of FET 560. Line 515 between rectifier 150 and FET 560 is also coupled to ground through capacitor 505 to stabilize the voltage.
Source terminal S of FET 560 is coupled to positive terminal Act+ of actuator 520. FET 560 acts as switch to provide power from three phase spin motor 509 to actuator 520 during a power failure. Gate G of FET 560 is coupled to drain D of FET 546. Source S of FET 546 is coupled to ground and gate G of FET 546 is coupled to "Power On/Reset" (POR) line 570. The voltage on POR line 570 is controlled by voltage monitor 528. Under normal operations POR line 570 is pulled high by voltage monitor 528; therefore, FET 546 conducts and pulls gate G of FET 560 and gate G of FET 542 low to prevent any power from three phase spin motor 509 reaching actuator 520. However, once a power failure occurs the voltage on POR line 570 goes low, which turns off FET 546 and allows gate G of FET 560 to be pulled high by auxiliary power supply 504 through resistor 544.
During a power failure, FET 542 which has gate G coupled to drain D of FET 546, source S coupled to ground, and drain D coupled to negative terminal Act- of actuator 520 through resistor 590, provides the ground path for actuator 520. Resistor 544 is coupled to auxiliary power supply 504 to pull gate G of FET 542 and FET 560 high during a power failure. FET 546, which has source S coupled to ground and gate G coupled to POR line 570, pulls gate G of FET 542 and FET 560 low during normal operations to prevent FET 542 from providing a ground path for actuator 520 and to prevent FET 560 from providing power from three phase spin motor 509 to actuator 520.
Hence there is a need for a method or a circuit to perform dynamic head unloading regardless of the position of the heads at the onset of a power failure. Specifically, the method or circuit must satisfy the conflicting need of greater unloading force when the heads are positioned near the outside diameter of the disk with the need of lower unloading force when the heads are positioned near the inner diameter of the disk.